Two major processes have been utilized to produce phosphoric acid from phosphate rock.
In the first process, the so-called "furnace process", mined phosphate rock is combined with coke and silica and reduced at high temperature in a furnace so as to produce elemental phosphorous. Phosphoric acid is then produced by burning the elemental phosphorous with air so as to produce P.sub.2 O.sub.5, and then absorbing the P.sub.2 O.sub.5 in water. The phosphoric acid produced by this process is of high purity and suitable for substantially all uses with little or no subsequent treatment. Unfortunately, however, the "furnace process" suffers from the disadvantage that it is a relatively expensive production process.
In the second process, the so-called "wet process" (and the one which is of concern here), mined phosphate rock is reacted with sulfuric acid in a reactor so as to produce phosphoric acid and calcium sulfate (gypsum). More particularly, the mined phosphate rock is typically first crushed in a ball mill (or rod mill) to increase its effective surface area, and then it is fed to a reactor. In the reactor, the phosphate rock is reacted with sulfuric acid, in a medium of phosphoric acid and gypsum, so as to produce additional phosphoric acid and gypsum. The phosphoric acid and gypsum are removed from the reactor as a slurry and passed along to a filter where the phosphoric acid is removed from the slurry. The remaining material in the slurry, primarily gypsum and acidic hot water, is then passed along to a gypsum pile which is located adjacent to a cooling pond. There the slurry is deposited so that the gypsum is added to the gypsum pile while the acidic hot water percolates down into the cooling pond. Thereafter, some of the pond water may be directed back to the production plant for use in cooling operations or in concentration control; the remainder of the acidic pond water is subsequently treated with lime to neutralize its acidity before being discharged into the fresh water systems of the surrounding area.
At one time, the mined phosphate rock was dry ground in the ball mill. However, it was subsequently recognized that wet grinding of the phosphate rock offers a number of advantages over dry grinding. First, dust pollution is largely eliminated. Second, rock drying (necessitated because the unground phosphate rock typically contains between 8 and 12% water by weight when received from the mine) is eliminated. Third, conveying the slurry produced by wet grinding is easier than conveying dry crushed rock. Fourth, metering the slurry produced by wet grinding is easier than metering dry crushed rock.
Accordingly, in U.S. Pat. No. 4,044,107, it was proposed that fresh water be introduced into the ball mill along with the phosphate rock so that wet grinding could be conducted. Thereafter, the fresh water and ground phosphate rock are passed out of the ball mill as a slurry for subsequent introduction into the reactor.
Unfortunately, however, the method proposed in U.S. Pat. No. 4,044,107 suffers from a number of drawbacks. First, it significantly increases the amount of fresh water consumed by the phosphoric acid production plant, since fresh water is required for the rock grinding operation. This can be a serious problem in certain areas, e.g. Florida, which at various times can suffer from a shortage of fresh water. Second, the method proposed in U.S. Pat. No. 4,044,107 suffers from the disadvantage that the introduction of fresh water into the grinding operation results in the production plant producing increased quantities of contaminated water, disposal of which presents a problem. More particularly, the total quantity of contaminated water produced by the production plant tends to increase when using the method proposed in U.S. Pat. No. 4,044,107 for two interrelated reasons. First, the introduction of fresh water into the grinding operation means that more fresh water enters the contaminating production system at the front end of the system, so that more contaminated water is produced on the back end of the system. Second, since the production plant must maintain certain minimum materials concentrations throughout the production process, the addition of fresh water to the system in the grinding operation means that less contaminated pond water can be recycled into the production process during process control. Accordingly, with more fresh water entering the production process and less pond water able to be recycled into the production process, the total quantity of contaminated water present in the cooling pond tends to grow. This contaminated pond water must eventually be neutralized with lime before it can be discharged into the fresh water systems of the surrounding area. Such lime treatment can be costly, particularly when required on a large scale.
U.S. Pat. No. 4,044,187 recognizes that one could substitute recycled contaminated pond water in place of the aforementioned fresh water for use in the wet grinding operation. Such a substitution would tend to solve both of the aforementioned difficulties (i.e., the problems of fresh water consumption and contaminated water production), but it in turn leads to new problems. In particular, the high acidity of the pond water (typically at a pH of between 1.5 and 2) makes it extremely corrosive to a number of the components ordinarily used to fabricate the ball mill, e.g. the forged steel balls and the liner made of a nickel/iron alloy. To forestall such corrosion, the ball mill either must be fabricated from different materials or, alternatively, the highly acidic pond water must be neutralized before being admitted into the ball mill. Neither arrangement is considered entirely satisfactory. In addition, using untreated pond water in the wet grinding operation tends to cause problems with fluorine evolution and with scaling of the rock slurry lines.
U.S. Pat. No. 1,894,514 teaches a concept closely related to the idea of using recycled contaminated pond water in the wet grinding operation. According to this patent, weak phosphoric acid is recycled from the recovery stages of the production plant to the ball mill for use in wet grinding operations. Such an arrangement offers the same benefits as using contaminated pond water for the wet grinding operation (i.e., the problems of fresh water consumption and contaminated water production are eliminated): however, since the weak phosphoric acid being recycled to the ball mill is essentially just a concentrated form of pond water (or, stated more correctly, usually pond water is essentially just a diluted form of weak phosphoric acid), substantially all of the problems introduced by the use of untreated pond water for grinding are encountered when using weak phosphoric acid for grinding, except perhaps that the problems occur on a larger scale.
U.S. Pat. No. 4,181,703 offers yet another technique for conducting wet grinding of the phosphate rock. According to this patent, salt water is used instead of fresh water for the grinding operation. Such a substitution is believed to solve the aforementioned problem of fresh water consumption, but it fails to reduce the total quantity of contaminated water produced by the plant. Thus, the costly problem of water treatment remains. In addition, the use of salt water in the grinding operation can lead to corrosion problems in the ball mill, as well as in other downstream elements in the phosphoric acid production plant. Furthermore, the use of salt water in the grinding operation could create still other problems by possibly interfering with essential chemical reactions occurring in the reactor.